Question on LTC2470

When using the LTC2470 for converting a mV signal into a binary value, I face an issue with the conversion result in case Vin is equal or higher than 0.5*Vref.The conversion result of all values below 0.5*Vref is as expected.

For instance an input voltage of 200mV is converted to approx. 10450 decimal value as is expected. However, 700mV is converted to approx. 23155 decimal value, which is not expected. Expected is about 36700 decimal value.

It doesn't seem to be a sampling mismatch in a way that the first (MSbit) is missed.

On the SPI bus a memory chip is also available, but the CS is not active during communication with the LTC2470.

Any idea what I'm doing wrong? ADC schematic part is included as png file. CPOL is 0.

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